Long-span lead screw assembly with anti-backlash nut

ABSTRACT

A lead screw assembly having a lead screw rotatable within a hollow tubular portion of a reinforcing rail. A nut engages with threads of the lead screw and is movable along the reinforcing rail. The lead screw includes plurality of first threaded portions having an outer diameter and extending lengthwise of the lead screw; and at least one second gap portion having an outer diameter that is less than the outer diameter of the first threaded portions. The second gap portion is positioned between adjacent first threaded portions of the lead screw. At least one generally U-shaped bearing is secured within the reinforcing rail and contacts the lead screw at a second gap portion. Each U-shaped bearing supports the lead screw along its length as the screw rotates within the rail, so that the lead screw assembly can be safely operated at high speeds over comparatively long distances while minimizing whipping and vibration of the lead screw. The reinforcing rail can include a base portion which permits the assembly to be securely mounted to a support structure at any convenient location along its length. 
     An anti-backlash nut assembly includes a threaded follower for engagement with the threads of a lead screw. A pair of wedges bias the follower in a radial direction so that the threads of the follower are brought into forcible engagement with the mating threads of the screw. In a preferred embodiment, the nut assembly is adjustable to provide variable levels of backlash resistance and wear-compensation.

BACKGROUND OF THE INVENTION

Devices and methods for bilateral motion of a load are known whichutilize a rotating lead screw and a threaded nut that is driven by thelead screw. In U.S. Pat. No. 6,422,101, issued Jul. 23, 2002 to thepresent inventors, the entire teachings of which are incorporated hereinby reference, a lead screw assembly is described which employs ananti-backlash nut assembly on a rotating lead screw. A hollow, elongatedreinforcing rail extends lengthwise of and surrounds the lead screw. Thereinforcing rail has a slot extending lengthwise of the central axis ofthe lead screw. The anti-backlash nut assembly has a nut whichcompletely surrounds the outer diameter of the reinforcing rail, and ismovable along the length of the reinforcing rail. The anti-backlash nuthas a pair of threaded tongue portions which extend radially through theslot to engage the threads of the lead screw for moving the nut inreciprocating motion lengthwise of the rail. A load or tool can bemounted to the nut for reciprocating motion along the length of theassembly.

A lead screw assembly such as described in U.S. Pat. No. 6,422,101 isparticularly advantageous for applications in which a high degree ofspacial resolution in the reciprocating motion of a load is desired.However, the operation of any lead screw device is limited by the“critical speed” of the screw shaft. Beyond a certain critical speed,the rotation of the screw within the reinforcing rail becomes unstable.This critical speed is a function of both the length of the assembly andthe diameter of the screw shaft. Thus, in the case of a four foot shaftwith a ⅜ inch outer diameter, for instance, the “critical speed” isapproximately 680 revolutions-per-minute (RPM). Beyond this speed,whipping and vibration forces become excessive, and the apparatus canself-destruct. In practice, this phenomena generally restricts theoperation of the lead screw assembly to lower speeds over relativelyshort spans. It would be desirable to provide a lead screw assembly thatcan successfully reciprocate a load at high speeds over relatively longdistances.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a lead screw assemblywhich comprises a lead screw rotatable about a central axis and areinforcing rail having a hollow tubular portion surrounding the leadscrew and a slot extending lengthwise of the central axis. The leadscrew includes plurality of first threaded portions having an outerdiameter and extending lengthwise of the lead screw; and at least onesecond gap portion having an outer diameter that is less than the outerdiameter of the first threaded portions. The second gap portion ispositioned between two first threaded portions of the lead screw. A nutis movable along the reinforcing rail, the nut having a tongue portionextending radially through the slot and engaging with threads on thelead screw. At least one generally U-shaped bearing is secured withinthe reinforcing rail and contacts the lead screw at a second gapportion. Each U-shaped bearing supports the lead screw along its lengthas the screw rotates within the rail, so that the lead screw assemblycan be safely operated at high speeds while minimizing whipping andvibration of the lead screw.

In another aspect, the a lead screw assembly comprises a threaded leadscrew and a reinforcing rail, where the reinforcing rail comprises ahollow tubular portion surrounding the lead screw and having a slotextending lengthwise of its central axis; and a base portion extendingfrom the hollow tubular portion opposite the slot, the base portionadapted to secure the reinforcing rail to a support structure. Theassembly further comprises a nut that is movable along the reinforcingrail, the nut having a threaded tongue portion extending radiallythrough the slot being engageable with the threaded lead screw, so thatthe nut is moved in reciprocating motion along the rail when the screwis rotated. The base portion of the reinforcing rail permits theassembly to be securely mounted to a support structure at any convenientlocation along its length. Furthermore, the nut need not extendcompletely around the exterior of the reinforcing rail, so that theassembly requires minimal clearance space for the movement of the nut,and can be made more compact.

In yet another aspect, the invention relates to an anti-backlash nutassembly comprising a nut body having a central cavity for engagementwith a reinforcing rail and a threaded follower within the nut body. Apair of wedges bias the follower in a radial direction so that thethreads of the follower are brought into forcible engagement with matingthreads on a lead screw. In a preferred embodiment, the nut assembly isadjustable to provide variable levels of backlash resistance andwear-compensation. In one aspect, the nut assembly is adjusted using apair of set screws, and optionally an elastic member, which engage thewedges and control the bias force on the follower.

In yet another aspect, the invention relates to a method for bilateraltranslation of a nut assembly, which comprises, inter alia, securing agap portion of a lead screw within a reinforcing rail using a generallyU-shaped bearing; and rotating the lead screw within the reinforcingrail to cause the nut assembly to move along the reinforcing rail.

The invention further relates to a method of operating an anti-backlashnut assembly which comprises providing a nut assembly with a threadedfollower and a pair of wedges, and pre-loading the wedges against thefollower to force the threads of the follower into forcible engagementwith mating threads of the lead screw. The bias force against thefollower is preferably adjusted using set screws and optionally anelastic member to provide variable levels of backlash-resistance andwear-compensation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1A is a plan view of a reinforced lead screw assembly according toone embodiment of the present invention;

FIG. 1B is a perspective view of a reinforced rail;

FIG. 2 is a perspective view of a section of the assembly of FIG. 1A;

FIG. 3 is a cross-sectional view of a reinforced lead screw assembly ofthe invention;

FIG. 4 is a side view of a generally U-shaped bearing according to oneaspect of the invention;

FIG. 5 is a perspective view of an anti-backlash nut assembly accordingto one aspect of the invention;

FIG. 6 is a side view of a threaded nut follower;

FIG. 7 is a side cross-section of an anti-backlash nut with threadedfollower;

FIGS. 8A–8C are top views of an anti-backlash nut assembly with variablebacklash control and wear resistance;

FIG. 9 is a top view of an anti-backlash nut assembly with wedgesdesigned to bias a follower to a side of the assembly;

FIGS. 10A–10B schematically illustrate an anti-backlash nut assembly inwhich the angle of interface between a threaded follower and a pair ofbias wedges is selected to be approximately equal to the angle of thethreads of a lead screw.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

A reinforced lead screw assembly 100 according to one embodiment of theinvention is seen assembled in plan view in FIG. 1A. A section of theassembly 100 between A—A and A′—A′ is shown in perspective view in FIG.2. The lead screw assembly 100 includes a lead screw 2 that is rotatableabout an axis, α. A projection 6 is formed on one end of the lead screw2 to connect it to a reversible driving motor (not shown) in order torotate the lead screw 2 alternatively in clockwise and counterclockwisedirection. The lead screw 2 can be made from any suitable material, suchas steel. The screw could also be aluminum, for example.

The screw 2 includes a plurality of first threaded portions 3 which runalong the length of the screw. The threaded portions 3 include threads 4having a first diameter. Between each of the first threaded portions 3are second gap portions 5. The gap portions 5 preferably include threads8 having a second diameter that is less than the diameter of the threads4 on the threaded portions 3. Alternatively, the gap portions 5 can beunthreaded. Contacting the screw shaft at each gap portion 5 is agenerally U-shaped bearing 21 which will be described in greater detailbelow.

An elongated reinforcing rail 9 extends lengthwise of the screw 2. (Thereinforcing rail 9 without a lead screw is shown in perspective view inFIG. 1B). The reinforcing rail 9 includes a hollow tubular portion whichreceives the lead screw 2, and a slot 10 extending lengthwise andparallel to the central axis α of the lead screw 2. The hollow tubularportion is defined by side walls 7, which may constitute splined bearingsurfaces. The lead screw 2 is journaled within the tubular portion ofthe reinforcing rail 9 by bearings 16 and 19 so that the screw may berotated relative to the reinforcing rail 9.

As shown in FIG. 2, the reinforcing rail 9 also includes a base portion11 which extends out from the hollow tubular portion opposite the slot10. The base portion 11 can be provided with fastening holes 17 forsecuring the apparatus to a support structure (not shown). In operation,the base portion 11 would be secured in a machine structure with areversible driving motor attached to the lead screw 2.

The reinforcing rail 9 can be made from any suitable material. In apreferred embodiment, the reinforcing rail is made from aluminum, whichcan be extruded into the shape of the rail 9.

A nut, which in this embodiment is an anti-backlash nut 34, is movablein bilateral direction along the reinforcing rail 9. In this embodiment,the nut 34 has a generally rectangularly-shaped body 35 that extendsover the hollow, tubular portion of the reinforcing rail 9. Inside thenut body is a threaded follower 40 (see FIG. 3) that extends into theslot 10 of the reinforcing rail 9 to engage with the threads 4 of therail, as will be described in greater detail below. The interior of thenut body includes bearing surfaces 36 (see FIG. 2), which can be madefrom a self-lubricating plastic material, and which are configured tomate with the splined side walls 7 of the reinforcing rail. A load ortool can be secured to the nut body in any convenient manner. The loadwith the anti-backlash nut 34 is reciprocated back and forth over thereinforcing rail 9 by the rotation of the reinforced lead screw 2.

As distinguished from conventional lead screw assemblies, such asdescribed in U.S. Pat. No. 6,422,101, the nut 34 in this embodiment doesnot extend completely around the outer circumference of the reinforcingrail 9. Instead, the nut 34 extends only over the hollow tubular portionof the rail 9, as defined by side walls 7. The rail 9 further includes awide base portion 11 extending out from the tubular portion opposite thenut 34. This is a particularly advantageous design, since it permits thereinforcing rail 9 to be securely mounted to a support structure via thewide base portion 11 at any convenient location along its length.Furthermore, since the nut 34 does not extend completely around theexterior of the reinforcing rail 9, the assembly requires less clearancespace for the movement of the nut 34. The lead screw assembly of theinvention is thus able to provide greater stability in a more compactdesign than comparative devices of the prior art.

As illustrated in FIGS. 1A and 1B, the side walls 7 of the reinforcingrail 9 are provided with a slot 15 proximate to one end of the rail.Bearing 16 includes a protrusion 18 which fits into the slot 15 toassist in anchoring the bearing 16 and screw 2 within the hollow portionof the rail 9. The slot 15 advantageously isolates the bearing 16 fromthe portion of the rail 9 over which the nut 34 travels. Thus, if thebearing 16 expands due to thermal expansion, for example, any expansionof the side walls 7 exterior to the bearing 16 will have no effect onthe portion of the rail 9 over which the nut travels. At the other endof the assembly, bearing 19 is not anchored to the rail 9. Thus, if thescrew 2 expands or contracts in length relative to the rail 9 due tothermal expansion effects, for instance, the free-floating bearing 19can move in conjunction with the expansion or contraction of the screw.In this way, stress and buckling of the screw 2 and/or the rail 9 can beavoided.

Turning now to FIG. 3, a cross-sectional view of a lead screw assembly100 with anti-backlash nut 34 according to one embodiment of theinvention is shown. The nut 34 is shown in engagement with a gap portion5 of the lead screw 2. As illustrated in FIG. 3, the outer diameter ofthe screw 2 at the gap portion 5 is significantly less than the outerdiameter of the threads 4 at the first threaded portions 3. Thereinforcing rail 9 includes a slot 12 that extends lengthwise along thebottom of the hollow portion of the rail 9 (see FIG. 2). The generallyU-shaped bearing 21 includes a protrusion 27 which fits into the slot 12at the bottom of the rail 9. The protrusion 27 thus anchors the bearing21 to the rail 9 to prohibit the bearing 21 from rotating relative tothe rail 9. It will be understood that any convenient means foranchoring the bearing 21 within the rail 9 can be employed. For example,the rail 9 could include a protrusion which fits within a correspondingnotch on the bearing.

The gap portion 5 of the lead screw 2 is journaled within the U-shapedbearing 21. The U-shaped bearing 21, being anchored to the rail 9 byprotrusion 27 and slot 12, supports the lead screw 2 within the rail 9,while permitting the screw 2 to rotate within the bearing 21.Preferably, as shown in FIGS. 1 and 2, this gap portion 5 and U-shapedbearing 21 arrangement is repeated at various locations along the lengthof the assembly. At each location, the U-shaped bearing 21 providesadditional support to the lead screw 2 as it rotates within the rail 9,thus minimizing any whipping or vibration of the rotating screw. Theadditional intermittent support provided by U-shaped bearings 21effectively decouples each of the first threaded portions 4 from oneanother as the screw 2 rotates within the rail 9. Thus, the assembly canbe operated over effectively longer spans at high rates of speed withoutencountering instability.

By way of example, a four foot long shaft having an outer diameter of ⅜inches is normally limited by a “critical speed” of about 680 RPM.However, when the threaded shaft is intermittently necked and supportedby U-shaped bearings 21 spaced at about 4 inch increments, as describedabove, the critical speed is greatly increased, and the apparatus can besafely operated at speeds of up to about 3000 RPM or more.

A U-shaped bearing 21 according to the invention is shown in greaterdetail in FIG. 4. In general, the bearing 21 includes a protrusion 27and a pair of curved arms 21 extending upwards from the protrusion 27.The outer diameter of the curved arms 21 is approximately equal to theinner diameter of the side walls 7 of the reinforcing rail 9. The innerdiameter of the bearing arms 21 is approximately equal to the outerdiameter of the gap portion 5 of the lead screw 2. The bearings arepreferably made from a self-lubricating plastic material, such as acetyland graphite.

The operation of the lead screw assembly will now be described withreference to FIG. 3. As shown in FIG. 3, the follower 40 of theanti-backlash nut 34 has a tongue portion 44 that extends down from thenut 34, through the slot 10 of the reinforcing rail 9. The tongueportion 44 includes threads 46 on its bottom surface. The threads 46engage mating threads 4 on a first threaded portion 3 of the lead screw2, such that when the screw 2 is rotated, the rotation of the lead screwthreads 4 with respect to the follower threads 46 causes the nut 34 tomove along the length of the screw. The direction of the screw'srotation (i.e. clockwise or counterclockwise) determines the directionin which the nut is translated. The nut 34 is guided along the rail 9 bythe splined side walls 7 of the rail 9, which mate with bearing surfaces36 on the interior of the nut 34.

Preferably, the threaded tongue portion 44 of the nut has a length thatis greater than the length of the gap portions 5 of the lead screw 2, sothat as the nut traverses along the length of the screw, a substantialportion of the threaded tongue 44 always remains engaged with one ormore of the first threaded portions 4. However, when the nut 34 ispartially over a gap portion 5 of the screw, the threads 46 of thetongue 44 can engage with smaller-diameter threads on the gap portion 5of the screw. Alternatively, the gap portion 5 of the screw 2 may beunthreaded, in which case the nut 34 is driven exclusively by theengagement of the mating threads 4 and 46 on the first portion 3 of thescrew and the tongue portion 44 of the follower 40, respectively.

As shown in FIG. 4, the curve side walls 23 of the U-shaped bearing 21are sized to permit the tongue portion 44 of the nut follower 40 (shownin phantom) to freely travel over the gap portion.

FIG. 5 is a perspective view of an anti-backlash nut assembly 34according to one aspect of the invention. The assembly 34 includes agenerally rectangularly shaped body 35. The body 35 includes a hollowinterior portion 60 with a pair of bearing surfaces 36 configured tomate with a reinforced guide rail 9, as described above. The body 35also includes a generally oval-shaped opening 62 at the top of the body.At one end of the opening is a large, arcuate wedge 38, and at theopposite end is a U-shaped stop, or U-stop 50, with arms 51 extendingaround the circumference of the opening 62 to the large wedge 38. Thetop of the body 35 further includes a recessed portion 54 for receivinga cap (see 52 in FIG. 3). The front and rear faces of the body 35 caneach include holes 70 for a set screw, the function of which will bedescribed in detail below.

The anti-backlash nut assembly 34 also includes the aforementionedfollower 40 which engages with the threads of a lead screw, aspreviously described in connection with FIG. 3. The follower 40 ispositioned within the oval-shaped opening 62 of the nut body 35, so thatthe threaded tongue portion 44 of the follower extends into the hollowinterior portion 60 of the body 35. An exemplary threaded nut follower40 is shown in side view in FIG. 6. The follower 40 includes a flat topsurface 43, and two sloping surfaces 45,46 extending from the topsurface 43 to flat ledges 42, 41 at either end of the follower. Thebottom of the follower includes tongue portion 44, which has a threadedbottom surface 46 for engagement with mating threads on a lead screw(not shown).

The fully assembled anti-backlash nut assembly is shown incross-sectional side view in FIG. 7. As shown, the assembly includes thefollower 40 mounted within the nut body 53 between the large wedge 38and the U-stop 50. Between the follower 40 and the U-stop 50 is asecond, small wedge 56. Large wedge 38 and small wedge 56 have slopedsurfaces that abut the sloped surfaces 45, 46 of the follower.Preferably, the angles of the mating sloped surfaces of the wedges andthe follower are supplementary with respect to one another. The bottomsurfaces of large wedge 38 and small wedge 56 rest on the flat ledges42, 41 of the follower. Cap 52 is secured across the top of the body 35to secure the follower and wedges between the cap and the lead screw.

In operation, the wedges 38, 56 can be pre-loaded against the follower40 such that the force from the sloped surfaces of the wedges 38, 40continually forces the follower 40 down into engagement with the leadscrew. Since the follower 40 is biased in a radial direction towards thelead screw, the threads 46 of the follower are continually forced intofirm engagement with the flanks of the threads 4 of the lead screw. Thisforcible engagement of the mating threads prevents backlash while theanti-backlash nut translates bilaterally along the reinforcing rail 9.

According to one aspect of the invention, the anti-backlash nut isadjustable to provide for variable degrees of both backlash control andwear-resistance. FIGS. 8A–8C are top views of an anti-backlash nutassembly 34 according to this preferred embodiment of the invention. Thecap 52 is not shown to better illustrate the components of the assembly,though in operation the cap 52 would be present to secure the wedges andfollower within the nut body.

As shown in FIG. 8A–8C, the nut assembly 34 can be adjusted to providevariable degrees of backlash resistance and wear compensation. As can beseen in the figures, the assembly includes a pair of set screws 71 and72 in respective holes 70, 76 of the nut body 35. A first set screw 71in first hole 70 is threaded-through U-stop 50 and can contact againstsmall wedge 56. A second set screw 72 is located in second hole 76, andcontacts against an elastic spring member 73, which is preferably madefrom an elastomeric rubber material. The elastic spring member couldalso be a wire spring, for example. The spring member 73 is also locatedin hole 76, and abuts against the large wedge 38. By adjusting thepositions of the two set screws 71, 72, the pre-load applied by thewedges 38, 56 to the follower 40 can be adjusted to provide variablelevels of backlash resistance and wear compensation.

In FIG. 8A, for instance, the set screws 71, 72 are adjusted to provideno pre-load between the wedges 38, 56 and the follower 40. Therefore,there is no anti-backlash function and no wear compensation with thissetting. This would be desirable, for instance, in applicationsrequiring low torque where a certain degree of slack or backlash betweenthe threads of the follower and the threads of the screw is tolerable.

The assembly of FIG. 8A can be easily adjusted to provide ananti-backlash function, as shown in FIG. 8B. Here, the first set screw71 is adjusted to back the small wedge 56 away from the large wedge 38.Next, the second set screw 72 is adjusted to move the spring member 73tight against the large wedge 38. This pre-loads the large wedge 38heavily against the U-stop 50, thus eliminating the gap 75 between thesecomponents (see FIG. 8A). The first set screw 71 can then be adjusted tovary the pre-load between the wedges 56, 38 and the follower 40. Thepre-load between these components determines the degree ofbacklash-resistance for the nut.

In a third example, shown in FIG. 8C, the variable backlash-resistancecan be combined with a wear-compensation function. Wear-compensation maybe desirable for applications in which the nut will have an extendedlength-of-service. As the nut operates over a prolonged period of time,wear will inevitably occur in the threads of the nut, thus increasingthe backlash of the system. This can be easily compensated for as shownin FIG. 8C. Here, the first set screw 71 is moved forward to create agap 75 between the U-stop 50 and the large wedge 38. The second setscrew 72 is also moved forward to bias the elastic member 73 against thelarge wedge 38. In this mode, the wedges 38 and 56 are both pre-loadedagainst the follower to provide backlash resistance. In addition, sincethe large wedge 38 is biased against the follower 40 by elastic member73, any wear to the threads of the follower 40 will be compensated forby the force of the large wedge 38 against the follower 40. The largewedge 38 will thus continue to bias the follower 40 radially downward totake up any slack in the threads due to wear. This wear compensationwill continue over time until eventually the large wedge 38 abuts theU-stop 50. In this mode, the gap 75 between the large wedge 38 and theU-stop 50 determines the amount of wear-compensation of the system.

Various other modifications can be made to the anti-backlash nut of thepresent invention. For instance, as shown in FIG. 9, the follower 40 andwedges 38 and 56 can be designed to bias the follower to a side of thenut body 35. Here, the wedges 38 and 56 are formed at an angle, so thatwhen the are pre-loaded against the follower 40, the follower is pushedagainst one side of the U-stop. This further aids in minimizing backlashbetween the respective threads of the follower and the lead screw.

According to yet another aspect, the angle(s) of sloped surfaces of thefollower 40 and wedges 38, 56 can be selected to have approximately thesame angle as the flanks of the threads of the lead screw. FIG. 10Ashows a plurality of threads 46 on the follower in engagement withmating threads 4 on a lead screw. FIG. 10B shows the follower 40 underbias by wedge members 38 and 56. By selecting the angle contact betweenthe wedges 38, 56 and the follower 40 to be approximately equal to theangle of contact between the mating threads 4 and 46, the vector forcesof the anti-backlash nut assembly can be effectively balanced. Thishelps to prevent the forces from pushing the follower away from the leadscrew, and assists in maintaining secure engagement between the matingthreads of the follower and the screw.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A lead screw assembly, comprising: a lead screw rotatable about acentral axis and comprising: a plurality of first threaded portionshaving an outer diameter and extending lengthwise of the lead screw; andat least one second gap portion having an outer diameter that is lessthan the outer diameter of the first threaded portions, the second gapportion being positioned between two adjacent first threaded portions; areinforcing rail comprising a hollow tubular portion surrounding thelead screw and having a slot extending lengthwise of the central axis;and at least one generally U-shaped bearing secured within thereinforcing rail and contacting the lead screw at a second gap portion.2. The lead screw assembly of claim 1, further comprising: a nutassembly having a nut movable along the reinforcing rail, the nut havinga tongue portion extending radially through the slot and having threadsengageable with the first threaded portions of the lead screw for movingthe nut in reciprocating motion lengthwise of the rail when the screw isrotated.
 3. The lead screw assembly of claim 2, wherein the nut assemblycomprises an anti-backlash nut.
 4. The lead screw assembly of claim 1,wherein the reinforcing rail comprises a base portion extending from thehollow tubular portion of the reinforcing rail opposite the slot, thebase portion adapted to secure the reinforcing rail to a supportstructure.
 5. The lead screw assembly of claim 2, wherein the hollowtubular portion of the reinforcing rail comprises a pair of side wallshaving splined outer surfaces, the nut assembly having internal bearingsurfaces configured to made with the splined outer surfaces of theside-walls.
 6. The lead screw assembly of claim 5, wherein the ends ofthe lead screw are secured within the hollow tubular portion of thereinforcing rail by a pair of end bearings which surround the shaft ofthe lead screw, and permit the screw to be rotated within the rail. 7.The lead screw assembly of claim 6, wherein a first end bearing includesa protrusion which fits into a slot in the hollow tubular portion of thereinforcing rail to anchor the bearing within the rail.
 8. The leadscrew assembly of claim 7, wherein the slot in the hollow tubularportion extends substantially completely around the circumference of thehollow tubular portion to mechanically isolate the portion of the hollowtubular member containing the first end bearing from the portion of thehollow tubular member over which the nut assembly travels.
 9. The leadscrew assembly of claim 7, wherein the second end bearing is notanchored to the reinforcing rail, and can move within the rail toaccommodate relative changes in the length of the lead screw or therail.
 10. The lead screw assembly of claim 1, further comprising meansfor anchoring the generally U-shaped bearing within the reinforcingrail.
 11. The lead screw assembly of claim 10, wherein the anchoringmeans comprises a protrusion on the generally U-shaped bearing whichfits into a recess on the interior of the hollow tubular portion of thereinforcing rail.
 12. The lead screw assembly of claim 11, wherein therecess comprises a slot which runs lengthwise along the interior of thehollow tubular portion.
 13. The lead screw assembly of claim 12, whereinthe generally U-shaped bearing supports the gap portion of the leadscrew so as to permit stable operation of the assembly at a speed thatwould otherwise be in excess of the “critical speed” of the lead screwassembly.
 14. The lead screw assembly of claim 1, wherein the lead screwcomprises steel.
 15. The lead screw assembly of claim 1, wherein thereinforcing rail comprises aluminum.
 16. A lead screw assembly,comprising: a threaded lead screw rotatable about a central axis; areinforcing rail, comprising: a hollow tubular portion surrounding thelead screw and having a slot extending lengthwise of the central axis;and a base portion extending from the hollow tubular portion oppositethe slot, the base portion adapted to secure the reinforcing rail to asupport structure; and a nut assembly having a nut movable along thehollow tubular portion of the reinforcing rail, the nut having a tongueportion extending radially through the slot and having threadsengageable with the first threaded portions of the lead screw for movingthe nut in reciprocating motion lengthwise of the rail when the screw isrotated.
 17. The lead screw assembly of claim 16, wherein the hollowtubular portion of the reinforcing rail comprises a pair of side wallshaving splined outer surfaces, the nut assembly having internal bearingsurfaces configured to made with the splined outer surfaces of theside-walls.
 18. The lead screw assembly of claim 17, wherein the nutassembly extends substantially around the outer circumference of thehollow tubular portion, but not around the base portion of thereinforcing rail.
 19. The lead screw assembly of claim 16, wherein thebase portion comprises at least one flat surface extending at leastpartially along the length of the rail for securing the rail to asupport structure.
 20. An anti-backlash nut assembly, comprising: a nutbody having a central cavity for engagement with a reinforcing rail, thenut body comprising: a follower within the nut body and having a tongueportion extending into the central cavity, the tongue portion havingthreads for engagement with a threaded lead screw rotatable within thereinforcing rail, the follower having first and second sloping surfacesat opposite ends of the follower; a first wedge within the nut body, thefirst wedge having a sloping surface for engagement with the firstsloping surface of the follower; and a second wedge within the nut body,the second wedge having a sloping surface for engagement with the secondsloping surface of the follower.
 21. The anti-backlash nut assembly ofclaim 20, wherein the first wedge and the second wedge are preloadedagainst the sloping surfaces of the follower to force the threads of thetongue portion into forcible engagement with mating threads of a leadscrew.
 22. The anti-backlash nut assembly of claim 21, wherein theamount of pre-load between the wedges and the follower is adjustable tovary the level of backlash-resistance of the assembly.
 23. Theanti-backlash nut assembly of claim 20, further comprising a springmember within the nut body, the spring member configured to apply anadjustable bias force to the first wedge.
 24. The anti-backlash nutassembly of claim 20, further comprising a pair of set screws within thenut body, the set screws configured to adjust the relative positions ofthe wedges within the nut body.
 25. The anti-backlash nut assembly ofclaim 20, further comprising a cap which encloses the follower, thefirst wedge, and the second wedge within the nut body.
 26. Theanti-backlash nut assembly of claim 20, wherein the nut body comprises agenerally oval-shaped opening extending from a first surface of the nutbody to the central cavity, the follower, the first wedge, and thesecond wedge being housed within the generally-oval shaped opening. 27.The anti-backlash nut assembly of claim 26, further comprising a U-stopat a first end of the generally oval-shaped opening and extendingpartially around the interior of the generally oval-shaped opening. 28.The anti-backlash nut assembly of claim 27, wherein the first wedge ispositioned at a second end of the generally oval-shaped opening,opposite the U-stop, and the second wedge is positioned within thegenerally oval-shaped opening between the U-stop and the follower. 29.The anti-backlash nut assembly of claim 28, further comprising: a firstset screw within the nut body, the first set screw configured to adjustthe position of the first wedge; and a second set screw within the nutbody, the second set screw configured to adjust the position of thesecond wedge.
 30. The anti-backlash nut assembly of claim 29, furthercomprising a spring member within the nut body, the spring member beingpositioned between the first set screw and the first wedge.
 31. Theanti-backlash nut assembly of claim 30, wherein spring member is adaptedto bias the first wedge against either the follower or the U-stop toprovide variable levels of backlash resistance and wear compensation.32. The anti-backlash nut assembly of claim 30, wherein the springmember comprises an elastomeric material.
 33. The anti-backlash nutassembly of claim 20, wherein the angle of the first sloping surface ofthe follower and the angle of the sloping surface of the first wedge aresupplementary.
 34. The anti-backlash nut assembly of claim 20, whereinthe angle of the second sloping surface of the follower and the angle ofthe sloping surface of the second wedge are supplementary.
 35. Theanti-backlash nut assembly of claim 20, wherein the angles of interfacebetween the sloped surfaces of the follower and the wedges are selectedhave approximately the same angle of interface as between the flanks ofthe threads on the lead screw and the follower.
 36. The anti-backlashnut assembly of claim 28, wherein the wedge members are configured tobias the follower against a side surface of the generally oval-shapedopening to further minimize backlash between the threads of the followerand the threads of a lead screw.
 37. A method for bilateral translationof a nut assembly, comprising: providing a lead screw having a pluralityof first threaded portions having an outer diameter and extendinglengthwise of the lead screw, and at least one second gap portion havingan outer diameter that is less than the outer diameter of the firstthreaded portions, the second gap portion being positioned between twoadjacent first threaded portions; providing a reinforcing railcomprising a hollow tubular portion surrounding the lead screw andhaving a slot extending lengthwise of the central axis; securing thesecond gap portion of the lead screw to the reinforcing rail with agenerally U-shaped bearing; providing a nut assembly having a nutmovable along the reinforcing rail, and a threaded portion engageablewith the threads of the lead screw; and rotating the lead screw withinthe reinforcing rail to cause the nut assembly to move along thereinforcing rail.
 38. The method of claim 37, wherein the generallyU-shaped bearing supports the gap portion of the lead screw in such amanner as to permit the lead screw to be rotated at a speed that wouldbe in excess of the “critical speed” of the lead screw assembly if thegenerally U-shaped bearing was not present.